The technical field of the present disclosure generally relates to control of selective catalytic reduction (SCR) aftertreatment systems for internal combustion engines.
SCR systems present several control challenges for internal combustion engine applications, including for mobile applications. SCR systems include a reduction catalyst and a reductant, such as urea or ammonia. An injector provides the reductant to the exhaust stream at a position upstream of the reduction catalyst, and the reductant enters the gas phase of the exhaust stream as ammonia. A delay sometimes occurs between the introduction of the reductant and the availability of the reductant product, for example injected particles of the reductant may need to evaporate into the exhaust stream, hydrolyze from urea to ammonia, and/or thoroughly mix into the exhaust stream for general availability across the reduction catalyst. Additionally, the reductant catalyst may include some ammonia storage capacity. Storage capacity can complicate the controls process, for example by creating additional controls targets (e.g. a storage target), by releasing ammonia unexpectedly (e.g. when a system condition causes a decrease in storage capacity), and/or by adsorbing some of the injected ammonia in an early part of the catalyst thereby reducing the availability of ammonia at a rear portion of the catalyst during catalyst filling operating periods.
The challenges presented by presently available SCR systems are exacerbated by the transient nature of mobile applications. The engine load and speed profile varies during operations in a manner that is determined by an operator and generally not known in advance to the SCR control system. Additionally, available feedback control systems suffer from several drawbacks. For example, the concentration of ammonia is difficult to determine in real time. Commercially reasonable NOx sensors can suffer from cross-sensitivity with ammonia, complicating the determination of the amount of NOx present in the exhaust gas outlet from the SCR catalyst. Further, ammonia is generally an undesirable constituent of the final exhaust emissions, and ammonia that is emitted from or “slips” from the catalyst represents ineffectively utilized reductant that increases operating costs. Therefore it is desirable to operate at a very low or zero ammonia concentration at the outlet of SCR catalyst outlet. However, NOx sensors that are cross-sensitive to ammonia hinder the ability to provide a reliable estimate of the amount of ammonia slip, reducing the effectiveness of feedback SCR control in providing an optimal amount of ammonia to the exhaust system and potentially creating false indications of an SCR and/or reductant injector fault conditions. As a result, further contributions in the detection and determination of ammonia slip conditions in SCR control are needed.